Headsets have long been used in conjunction with modern telephone equipment. Typically, headset users are ones whose jobs require either that they spend a substantial amount of time on the phone, or that their hands be free to perform other tasks. Examples of occupations where headsets are commonly used, include, among others, telephone operators, office receptionists, airline reservation clerks, stock brokers, customer service representatives, and police and fire department dispatchers.
The use of a headset provides many advantages. For example, a headset allows the user to perform other tasks while on the telephone. Examples of such tasks include taking messages, routing incoming calls, and using a computer terminal to enter or retrieve data. The use of a headset also reduces the amount of time required to answer an incoming call, thus making the headset user more efficient. In addition, the use of a headset eliminates the physical discomfort that can occur when a person talking on the telephone attempts to prop a regular telephone handset against his or her ear by tilting the head and raising the shoulder.
However, there are also disadvantages associated with the use of headsets. Since the headset's speaker is held in or against the user's ear, it is impossible to respond to any drastic change in volume by simply moving the speaker closer to, or further away from the ear as one typically does with a regular telephone handset. This is a problem for two reasons. First, incoming calls have different intensity levels, or volumes in common parlance. The volume varies from one call to the next depending on the person speaking, the telephone equipment and other conditions. In addition, there are occasional bursts of noise on the phone line. As those skilled in the art know, many voice signals in telephone calls will be received at a level of approximately -20 dBm. Noise bursts on the order of -10 dBm or -5 dBm are thus 10 to 15 dB higher than the average level for a voice signal.
If a calling party activates the dual tone multi-frequency (DTMF) (touch tone) keypad while his or her telephone station is connected to an operator wearing a headset, a touch tone signal having a level on the order of -3 dBm to 0 dBm can be generated on the telephone line at the operator's station.
It is well known that excessive noise causes fatigue and difficulty in concentration on one's work. This is particularly true if one experiences bursts of loud noises. Thus, a reduction in exposure to loud noises is desirable for both the comfort of employees wearing telephonic headsets and to prevent such employees from being unnecessarily fatigued and to meet the requirements of the Occupational Safety and Health Administration (OSHA).
There are existing headset control circuits which address this problem. The first type is a peak limiting circuit which suppresses extremely loud signals. A peak limiting device reduces the level of signals which exceed a predetermined level. The circuit does not increase or decrease the volume of signals which are below that level. As a result, most voice signals pass through the circuit unchanged. Only those signals (voice or noise) which are extremely loud are attenuated by the peak limiter. This type of device prevents the headset user from suffering discomfort or injury which could be caused by excessively loud signals. However, this approach does not bring quiet, low level signals up to a volume level at which they may be clearly and comfortably understood by the headset user. Additionally, most limiting circuits are clipping devices and simply clip off the excursion of a signal past a particular threshold causing odd harmonic distortion which is known to have a harsh sound to the listener.
Conventional automatic gain control (AGC) works in a well known manner to make the gain at a given stage of amplification a function which is inversely proportional to the signal level at a given point in the circuit. Most conventional AGC circuits simply feed back the output of a particular stage to provide a control signal which reduces the gain as the output increases. In most applications of AGC circuits to audio signal paths, the AGC is simply a form of negative feedback and is most commonly used to provide a non-linear (normally approximating logarithmic) signal level to gain characteristic.
It is well known in the art to use such an AGC circuit, together with some form of peak limiting or clipping arrangement, to prevent extremely loud sounds from making it through audio signal paths, particularly signal paths connected to headsets of telephone operators.
As is well known to those skilled in the art, automatic gain control circuits are devices which almost always respond to some form of integrated or average signal level. Those which respond very quickly may be thought of to be devices having very short integration times, and thus are circuits which average a very short time window of the signal level. Slower responding devices make the gain a function of the history of the signal over a longer most recent interval, i.e., they have a longer integration time. Those skilled in the art know that it is common to define and describe two characteristics of automatic gain control circuits known as attack time and release time. The attack time is the time period required after a sudden increase in the input signal amplitude for the gain of the AGC circuit to reach a predetermined percentage of the steady state change in gain it will make in response to continued application of the new input signal level. It may be likened to, and is often the equivalent of, a charging time constant for an R/C circuit. Similarly, release time is defined as a similar interval for the change in amplification which results from a sudden decrease in input signal level. Those skilled in the art know that many AGC circuits are designed so that they have differing attack and release times in order to achieve the desired goals of the AGC circuit without introducing an excessive amount of unpleasant distortion into the audio signal.
Naturally, the goal of AGC circuits is some form of dynamic range compression. Range compression is the act of reducing the difference in signal between the loudest and softest signals present in the signal path. In most applications for audible signals, including those in the art of telephony, it is desirable to have some form of relatively fast attack and slow release characteristics present in the AGC for the following reasons. The relatively fast attack time is desirable so that a loud signal will be attenuated rapidly enough to prevent acoustic shock to a headset wearer. A relatively slow release, usually on the order of one or more orders of magnitude greater than the attack time, is employed to prevent the phenomenon commonly referred to as "pumping" or "breathing". Pumping and breathing in AGC circuits is a phenomenon cause by too rapid a release time such that the listener hears the noise level rise rapidly during the short pauses in the audio signal such as pauses between words or brief quiet passages in music. A rapid increase in the gain due to a fast release time causes the gain to be turned up rapidly when the signal strength drops off, causing a sound (the increase in background noise) which reminds many listeners of a person exhaling. Hence, the expression "breathing".
It is well known to those skilled in the art that automatic gain control can be too successful in a manner which removes so much of the dynamic range of a speech signal that its starts to sound, mushy and distorted, and is difficult to understand.
In a typical voice grade telephone circuit in the public switched telephone network (PSTN) the noise floor for the channel is at a level of approximately -60 dBm. Good voice signals normally have a signal level of -15 to -25 dBm and poor ones will often be on the order of -40 to -50 dBm. Exceptionally good signals can be of a higher level, and DTMF signals on the line will often be on the order of 0 dBm. Therefore, in prior headset AGC circuits for telephone operators, it was prudent to employ peak limiting or clipping circuits, and undesirable to employ a relatively high level of maximum gain to bring the weakest voice signals up to a desired level. This limitation comes from the nature of conventional negative feedback AGC circuits, and in particular the fact that their quiescent no-signal condition is to be in a state in which the maximum gain is provided. An abrupt initiation of a high level signal can cause a loud transit to be applied to an operator's headphones. While a clipping circuit will effectively prevent the signal level from being damaging to the ear, it significantly distorts the signal and, as noted above, introduces distortion which adds to user fatigue.
Therefore, there is a need in the art for an improved automatic gain control circuit for headsets which provides a clear, comfortably audible signal over a rapidly changing range of input levels while eliminating both the "pumped" sound and any excessively loud signals. Additionally, there is a need for an AGC circuit for use with telephonic headphones which is forced to a quiescent state of a gain less than the maximum gain obtained from the AGC amplifier after a significant period of time is passed since the last audio signal above a predetermined noise threshold was applied to the circuit.